Add black_box to benchmarks.

[CathalMullan]

Probably over-used black_box, but good enough for now.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #122 will degrade performances by 13.91%

_{Comparing 121-make-use-of-black_box-in-benchmarks (e63d355) with main (96fb53f)}

Summary

❌ 2 regressions ✅ 14 untouched benchmarks

:warning: _Please fix the performance issues or acknowledge them on CodSpeed._

Benchmarks breakdown

	Benchmark	main	121-make-use-of-black_box-in-benchmarks	Change
❌	matchit benchmarks/wayfind	10.2 µs	11.5 µs	-10.66%
❌	path-tree benchmarks/wayfind	48.4 µs	56.2 µs	-13.91%

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:white_check_mark: All tests successful. No failed tests found.